Include USE INDEX hint with channel queries

The channel queries rely on the channel index being picked by the query service, but we should specify a USE INDEX hint, which forces the index to be used, or fail if it doesn't exist.

If a primary index happens to be defined on the bucket (maybe for dev-time purposes), the query service will randomly pick between the channel index and the primary index when issuing a channel query, which will impact performance non-deterministically.

This can be reproduced sporadically by running TestCoveringQueries when a primary index is defined on the bucket.